Make screenshots
of webpages, and save them locally or upload them to the cloud. This
feature will undergo A/B testing and will not be visible for some users.

Fine-tune your browser performance from the Preferences/Options page.

Sidebar (bookmarks, history, synced tabs) can now be moved to the right edge of the window

Search with any installed search engine directly from the location bar

Changed

The Flash plugin is now click-to-activate by default and only
allowed on http:// and https:// URL schemes. This change will be rolled
out progressively and so will not be visible to all users immediately.
For more information see the Firefox plugin roadmap

Breaking profile changes - do not downgrade Firefox and use a profile that has been opened with Firefox 55+.

Modernized application update UI to be less intrusive and more
aligned with the rest of the browser. Only users who have not restarted
their browser 8 days after downloading an update or users who opted out
of automatic updates will see this change.

Insecure sites can no longer access the Geolocation APIs to get access to your physical location